Damascene template for directed assembly and transfer of nanoelements

ABSTRACT

Damascene templates have two-dimensionally patterned raised metal features disposed on an underlying conductive layer extending across a substrate. The templates are topographically flat overall, and the patterned conductive features establish micron-scale and nanometer-scale patterns for the assembly of nanoelements into nanoscale circuits and sensors. The templates are made using microfabrication techniques together with chemical mechanical polishing. These templates are compatible with various directed assembly techniques, including electrophoresis, and offer essentially 100% efficient assembly and transfer of nanoelements in a continuous operation cycle. The templates can be repeatedly used for transfer of patterned nanoelements thousands of times with minimal or no damage, and the transfer process involves no intermediate processes between cycles. The assembly and transfer processes employed are carried out at room temperature and pressure and are thus amenable to low cost, high-rate device production.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. application Ser. No.14/356,328, filed May 5, 2014, which is the U.S. national phase of PCTApplication No. PCT/US2012/064176, filed Nov. 8, 2012, which claims thebenefit of U.S. Provisional Application No. 61/556,904 filed Nov. 8,2011, and U.S. Provisional Application No. 61/557,594 filed Nov. 9, 2011and entitled “Damascene Template for Directed Assembly and Transfer ofNanoelements”. Each of the aforementioned applications is herebyincorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention was developed with financial support from Grant Nos.EEC-0832785 and 0425826 from the National Science Foundation. The U.S.Government has certain rights in the invention.

BACKGROUND

Assembly of nanoelements on a template with precise alignment andorientation followed by transfer of the nanoelements to a recipientsubstrate is expected to accelerate large-scale production of nanoscaledevices. However, absence of highly versatile and reusable templates forhigh-throughput directed assembly and transfer with minimaldeterioration have hindered any progress.

Various templates fabricated through bottom-up or top-down processeshave been used to assemble nanoelements for achieving desiredarchitectures [1-3]. The template-guided fluidic assembly process isamenable to a variety of nanoelements and can result in high assemblydensity, yield and uniformity [4-6]. However, the assembly process isvery slow and hence not scalable. On the other hand, electrophoreticassembly involves assembling nanomaterials having a surface charge on aconductive surface over large areas (wafer scale) in a short period oftime [7-10]. When nanoelements are assembled by electrophoresis on atopographically patterned electrode with interconnected microscale andnanoscale features, due to the differences in potential drop at variousregions of the electrode, the assembly is non-uniform. Previously, thisimpediment has been circumvented by employing so-called “trenchtemplates” in which a lithographically-defined polymer pattern lies ontop of a uniform conducting layer guiding the assembly to the desiredlocations. Whenever assembled nanoelements in these trench templatesneeded to be transferred to a recipient substrate, the polymer has to beremoved, thereby limiting the template's use to a single assembly andtransfer cycle [11].

Transferring assembled nanoelements from one substrate to another whileretaining their two-dimensional order is a rather cumbersome processrequiring in-depth knowledge about the interaction energy betweendifferent materials and the nanoelements. Successful achievement ofordered nanoelement transfer onto flexible substrates would enable theproduction of various types of new devices such as thin-filmtransistors, gas sensors, and biosensors [12-14]. Even though transferof nanoelements using a template sacrificial layer (e.g. SiO₂ layer) hasbeen demonstrated for transfer onto flexible as well as rigidsubstrates, and with high transfer efficiency, such templates cannot bereused [15]. Intermediate sacrificial films such as PDMS and Revalphathermal tape for transferring nanoelements to the recipient substrateshave also been explored, but these introduce additional steps and henceresult in a complicated fabrication process leading to higher productioncosts [16-17].

SUMMARY OF THE INVENTION

The invention provides highly reusable, topographically flat damascenetemplates and methods for assembled nanoelements onto the damascenetemplates using electrophoresis. The invention also provides methods fortransferring assembled nanoelements from the damascene templates ontoflexible substrates using a “transfer printing” method. The transferprinting method can be used for fabricating microscale and nanoscalestructures, including combinations of microscale and nanoscalestructures on a single chip, without the need for an intermediate film[18-19].

The inventors have designed and fabricated topographically flatdamascene templates with submicron features using microfabricationtechniques together with chemical mechanical polishing (CMP). Thesetemplates are designed to be compatible with various directed assemblytechniques, including electrophoresis, with an essentially 100% assemblyand transfer yield for different nanoelements such as single-walledcarbon nanotubes and nanoparticles. These templates can be repeatedlyused for transfer thousands of times with minimal or no damage, and thetransfer process involves no intermediate processes between cycles. Theassembly and transfer processes employed are carried out at roomtemperature and pressure and are thus amenable to low cost, high-ratedevice production.

One aspect of the invention is a damascene template for theelectrophoretic assembly and transfer of patterned nanoelements. Thetemplate includes a substantially planar substrate, a first insulatinglayer, an optional adhesion layer, a conductive metal layer, a secondinsulating layer, and an optional hydrophobic coating. The firstinsulating layer is disposed on a surface of the substrate. The adhesionlayer, if present, is disposed on a surface of the first insulatinglayer opposite the substrate. The conductive metal layer is disposed ona surface of the adhesion layer opposite the first insulating layer, ordisposed on a surface of the first insulating layer opposite thesubstrate if the adhesion layer is absent. The second insulating layeris disposed on a surface of the conductive metal layer opposite theadhesion layer, or opposite the first insulating layer if the adhesionlayer is absent. The hydrophobic coating is selectively disposed onexposed surfaces of the second insulating layer opposite the conductivemetal layer; the hydrophobic coating is absent from the exposed surfacesof the conductive metal layer. The conductive metal layer is continuousacross at least one region of the substrate, or in some embodimentsacross the entire substrate. Within the region of the conductive metallayer, the conductive metal layer has a two-dimensional microscale ornanoscale pattern of raised features that interrupt the secondinsulating layer, leaving the second insulating layer to substantiallyfill the spaces between the raised features. The exposed surfaces of theraised features and the exposed surfaces of the second insulating layerare essentially coplanar, due to planarization by a chemical mechanicalpolishing procedure.

Another aspect of the invention is a nanoelement transfer system fortransfer of patterned nanoelements by nanoimprinting. The systemincludes the damascene template described above and a flexible polymersubstrate for reception of said plurality of nanoelements. In someembodiments the system also includes a thermally regulated imprintdevice for applying pressure between the damascene template and theflexible polymer substrate at a selected temperature above ambienttemperature.

Yet another aspect of the invention is a method of making the damascenetemplate described above. The method includes the following steps:

(a) providing a substantially planar substrate;

(b) depositing a first insulating layer on a surface of the substrate;

(c) optionally depositing an adhesion layer onto the first insulatinglayer;

(d) depositing a conductive metal layer onto the adhesion layer, or ontothe first insulating layer if the adhesion layer is absent;

(e) depositing a layer of lithography resist onto the conductive metallayer;

(f) performing lithography to create a two-dimensional pattern of voidsin the resist layer, whereby the surface of the conductive metal layeris exposed in the voids;

(g) etching the exposed surface of the conductive metal layer;

(h) removing the resist layer, leaving the entire surface of theconductive metal layer exposed, wherein the conductive metal layercomprises a two-dimensional pattern of raised features;

(i) depositing an insulating material to cover the conductive metallayer, including the raised features;

(j) removing the insulating material and a portion of the raisedfeatures by a chemical mechanical polishing method, whereby the raisedfeatures and insulating material are planarized, leaving atwo-dimensional pattern of raised features having exposed surfaces whichare coplanar with one another and with exposed surfaces of theinsulating material; and

(k) optionally silanizing selectively the exposed surfaces of theinsulating material with a hydrophobic coating of an alkyl silane.

In some embodiments, the method further includes the step of:

(l) chemically cleaning the exposed surfaces of the raised features ofthe conductive metal layer without substantially removing thehydrophobic coating on the insulating layer.

Still another aspect of the invention is a method of forming a patternedassembly of nanoelements on a damascene template. The method includesthe steps of:

(a) providing the damascene template described above;

(b) submerging the damascene template in a liquid suspension ofnanoelements;

(c) applying a voltage between the conductive metal layer of thedamascene template and a counter electrode in the liquid suspension,whereby nanoelements from the suspension are assembled selectively ontoexposed surfaces of the raised features of the conductive metal layer ofthe damascene template and not onto exposed surfaces of the secondinsulating layer of the damascene template;

(d) withdrawing the damascene template and assembly of nanoelements fromthe liquid suspension while continuing to apply voltage as in step (c);and

(e) drying the damascene template.

Another aspect of the invention is a method of transferring atwo-dimensional patterned assembly of nanoelements onto a flexiblepolymer substrate. The method includes the steps of:

(a) providing a patterned assembly of nanoelements on a damascenetemplate, such as one made by the method described above, and a flexiblepolymer substrate;

(b) contacting the patterned assembly of nanoelements with the flexiblepolymer substrate and applying pressure, whereby the patterned assemblyof nanoelements is transferred onto the flexible patterned substrate. Insome embodiments, step (b) is performed at a temperature above the glasstransition temperature of the flexible polymer substrate.

Still another aspect of the invention is a method of assembling andtransferring a two-dimensional patterned assembly of nanoelements onto aflexible polymer substrate. The method includes the steps of:

(a) providing the nanoelement transfer system described above and aliquid suspension of nanoelements;

(b) performing electrophoretic assembly of nanoelements from the liquidsuspension onto a damascene template according to the method describedabove to yield a patterned assembly of nanoelements on the damascenetemplate; and

(c) contacting the patterned assembly of nanoelements with the flexiblepolymer substrate and applying pressure, whereby the patterned assemblyof nanoelements is transferred onto the flexible patterned substrate. Insome embodiments, the step of contacting is performed at a temperatureabove the glass transition temperature of the flexible polymersubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a schematic of damascene template fabrication process.Resist was spun onto a Au/Si substrate and nanolithography was employedto define the patterns. Patterned Au was etched partially andplasma-enhanced chemical vapor deposition (PECVD) oxide (SiO₂) wasdeposited on the Au surface. Excess SiO₂ was removed by a chemicalmechanical polishing (CMP) process until the top surfaces of the Auraised features were revealed. Inset shows an artificially coloredcross-sectional scanning electron microscopy (SEM) micrograph exhibitingthe metal electrode (raised metal features) to be at the same height asthat of the SiO₂. FIG. 1B shows an optical image of a 3-inch damascenetemplate with high-resolution SEM images as the insets. FIGS. 1C.1,1C.2, and 1C.3 show simulated electric field strength results at 2.5 Vand pH 10.8 as a function of dishing amount. Electric field strengthnear the SiO₂ surface is of the same order of magnitude as that of Auelectrode while non-uniformity in the electric field from the edge tothe center of the electrode increases as the dishing amount increases.FIG. 1C.2 is top-viewed and FIG. 1C.3 is a cross-sectional view of theelectric field strength contours with 25 nm dishing amount. The contoursindicate that higher electric field strength is generated on theelectrode edges. FIG. 1D shows a schematic representation of across-section of an embodiment of a damascene template of the invention.The structures shown are: substrate (10), first insulating layer (15),adhesion layer (20), conductive metal layer (30), with raised features(40), second insulating layer (50), and hydrophobic coating (60).

FIG. 2 is a schematic of the assembly and transfer process usingdamascene templates. The insulating (SiO₂) surface of the damascenetemplate is selectively coated with a hydrophobic self-assembledmonolayer (SAM) of octadecyltrichlorosilane (OTS). Using electrophoresisnanoelements are assembled on the electrodes of the damascene template,which are then transferred to a flexible substrate using a printingtransfer method. After transfer the template is ready for the nextassembly and transfer cycle.

FIG. 3A shows a top viewed SEM micrograph of assembled silica particleson a damascene template. The applied voltage during assembly was 2 V,and the withdrawal speed was 1 mm/min. 100 nm silica particles assembledonly on the edge of gold electrode (shown in FIG. 1B). FIG. 3B is an SEMmicrograph of a typical high-density 100 nm silica nanoparticle assemblyresult for assembly at 2.5 V and withdrawal speed of 5 mm/min. Despitethe increase in withdrawal the assembly of nanoparticles was uniform.FIG. 3C shows 50 nm PSL particles assembled on the gold electrodesurface with high density using the same conditions as for 100 nm silicananoparticles. FIG. 3D shows 100 nm fluorescent silica particlesassembled on complex 2-D patterns demonstrating the versatility of thedamascene template. The inset figure is a fluorescence microscope imageof the assembled silica particles. FIG. 3E shows an SEM image of ahighly dense and organized single-walled carbon nanotube (SWNT) assemblythat was achieved at an applied voltage of 2.5V while the withdrawalspeed was maintained at 5 mm/min. FIG. 3F shows an SEM micrograph ofassembled SWNTs over a large scale exhibiting uniformity.

FIG. 4A shows SEM micrographs of a damascene template after transfer andFIG. 4B shows the transfer substrate with transferred SWNTs. FIG. 4C.1shows an optical micrograph of highly organized SWNT arrays with metalelectrodes on a PEN film. FIG. 4C.3 is an SEM micrograph and FIG. 4C.4is an AFM image of the transferred SWNT with two metal electrodes. Theatomic force microscopy (AFM) profile in FIG. 4C.2 shows that there areno indentations created on the transfer substrate. FIG. 4D shows I-Vcharacteristics for various channel length and fixed channel width (2.4μm). FIG. 4E shows the change in resistance of an SWNT channel (2.4 μmwidth and 30 μm length) as a function of bending radius of PENsubstrate.

FIG. 5A shows a schematic diagram of a conventional template used forelectrophoretic assembly. In this template, nanowire electrodes areconnected to a micron scale electrode, which is in turn connected to alarge metal pad through which the potential is applied. Shown in FIG. 5Bis a schematic of a damascene template in which both micron scale andnanometer scale electrodes are connected to a metal sheet underneath aninsulating layer. Corresponding equivalent resistor circuits are shownin both figures, where Rm is the resistance introduced due to the micronscale electrode, Rn is that of the nanoscale electrode, while Rs is thatof the solution. Shown in FIG. 5C are SEM micrographs of a typicalnanoparticle assembly result obtained for configuration shown in FIG.5A. Nanoparticles have assembled only on the micron scale electrodes andnot on the nanoscale electrodes which are connected to them.

FIGS. 6A.1, 6A.2, and 6A.3 show electric field simulation results forvarious thickness of electrode protruding from the insulating surface.As the electrode protruding height increases, the non-uniformity in theelectric field strength across the breadth of the electrode increasesdrastically, resulting in inconsistent assembly across the electrode.Shown in FIGS. 6B.1, 6B.2, and 6B.3 are the electric field simulationresults for the conventional template shown FIG. 5A. Irrespective of thethickness of the electrode, the non-uniformity in the electric fieldstrength still exists.

FIGS. 7A.1, 7A.2, and 7A.3 show AFM images and contour and FIG. 7A.4shows an SEM image of a damascene template in which the metal electrodesprotruded above the insulating surface by about 40 nm. FIGS. 7B.1, 7B.2,and 7B.3 show the AFM morphology of a flexible PEN substrate aftertransfer. Indented structures similar to those of the template appear onthe PEN substrate; the indented structures are about 40 nm deep.

FIGS. 8A and 8B show top-viewed SEM micrographs of damascene templatesafter assembly. The damascene templates shown were not treated with anOTS SAM layer. FIG. 8A shows nanoparticle assembly, and FIG. 8B showsSWNT assembly. The nanoelements were not specifically assembled onto themetal electrode but were also found on the insulator.

FIGS. 9A-9D show contact angle measurements of the OTS SAM coatedmetallic and insulating surfaces before and after treatment with piranhasolution. FIGS. 9A and 9B correspond to OTS SAM-coated SiO₂ surface(second insulating layer) before and after piranha solution treatment,respectively. The contact angle remained the same, establishing the factthat the OTS SAM layer remained intact. FIGS. 9C and 9D show an OTSSAM-coated gold surface before and after piranha solution treatment,respectively. A flat wafer with 150 nm thick gold layer sputtered ontoit was used for these measurements instead of a patterned substrate.After piranha treatment, the contact angle was reduced drastically,indicating that the OTS SAM layer was removed.

FIGS. 10A-10D show top-viewed SEM micrographs of a damascene templateafter SWNT assembly for various applied potentials: FIG. 10A 1.5 V; FIG.10B 2 V; FIG. 10C 2.5 V, and FIG. 10D 3 V. The rest of the assemblyparameters were kept constant. As can be seen from the images, assemblyefficiency on the metal electrode increases as a function of the appliedelectric field, and beyond a critical value the SWNTs begin to assembleeverywhere, including the insulator.

FIGS. 11A-11D show top-viewed SEM micrographs of a damascene templateafter SWNT assembly for various withdrawal speeds: FIG. 11A 3 mm/min;FIG. 11B 5 mm/min; FIG. 11C 10 mm/min, and FIG. 11D 15 mm/min. The restof the assembly parameters were kept constant. As can be seen from theimages, the efficiency of assembly on the metal electrode decreases withincreasing withdrawal speed, indicating the effect of removal momentacting on the SWNTs during withdrawal.

FIG. 12 shows a plot of the contact angle measurement of the OTS SAMcoated SiO₂ surface as a function of the number of assembly and transfercycles. The slope of the linear fit is ˜−0.18 indicating the robustnessof the damascene template in withstanding the wear and tear of multipleassembly and transfer cycles.

FIGS. 13A-13C show top-viewed SEM micrographs of a damascene templateafter transfer of SWNTs for various temperatures during the transfer:FIG. 13A 135° C.; FIG. 13B 150° C.; and FIG. 13C 160° C. The rest of thetransfer process parameters were kept constant. As can be seen from theimages, transfer efficiency (absence of SWNTs on the metal electrodeafter transfer) increases with increasing temperature, and ˜100%transfer is achieved at process temperatures higher than Tg (155° C.) ofthe flexible substrate (PEN) onto which it is transferred.

FIG. 14 shows a plot of the resistance measured as function of variousbending radii. The inset shows optical images of the experimental setupused to measure the electrical properties at the desired bending radius.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides methods for fabricating topographically flatdamascene templates for the assembly and transfer of nanoelements.Patterned assemblies of nanoelements such as nanoparticles and carbonnanotubes can be produced on the damascene template and transferred todesired locations on a receptor substrate, with resulting high densityand good uniformity of the patterned nanoelements. Transfer of assembledSWNTs or other nanoelements onto a flexible substrate can be performedwithout any intermediate steps and without a need for sacrificial films.The damascene templates of the invention are reusable and can beemployed in a high rate assembly and transfer process. In addition, thedamascene templates of the invention are compatible with various typesof nanoelements. The products and methods of the invention can providedrastic advancements in high rate manufacturing of flexible devices,such as electrical and optical devices, such as display devices andstrain guages, as well as biosensors and chemical sensors.

A schematic illustration of a process of damascene template fabricationis shown in FIG. 1A. Initially, a metallic layer (e.g., Au or W) isdeposited on an electrically insulating substrate, and lithography iscarried out to create a desired pattern for nanoelement assembly.Subsequently, partial etching of the metallic layer is conducted to formraised features having dimensions on the micrometer and/or nanometerscale. The raised features protrude above the plane of the rest of themetallic conductive layer. A thick layer of insulating material (e.g.,SiO₂ or SiN₄) is blanket deposited on these patterned structures. Achemical mechanical polishing (CMP) process is then carried out toremove the insulating material until it is essentially coplanar with thetop surfaces of the raised metal features, and until the top surfaces ofthe raised metal features are coplanar with one another across thesubstrate, or a portion of the substrate.

Thus, the resulting damascene template has nano/micro features connectedby a conductive film underneath an insulator (second insulating layer),which enables all of the micro/nano structures on the whole substrate,or a desired region of the substrate, to have equal potential duringelectrophoretic assembly. Preferred materials are gold for the metalliclayer and PECVD-deposited silicon dioxide for the insulating layer.

FIG. 1D shows a cross-sectional view of a damascene template embodimentof the invention. The substrate (10) is a base layer of electricallyinsulating material, such as silicon or a polymer. The substrate isessentially planar on at least one surface, or is entirely planar, andin some embodiments is substantially rigid, while in other embodimentsis flexible and can be bend to conform to a desired shape. The substratecan have any size or shape required for the particular application, butgenerally has a thickness of about 1 μm to about 10 μm, or about 100 μmor less, or about 1000 μm or less and a surface area on a planar surfaceof about 0.005 mm² or more, up to several cm². The substrate can befabricated from electrically insulating materials including silicon,silicon dioxide, organic polymers including epoxies and liquid crystalpolymers, or a photoresist material such as SU-8. The first insulatinglayer (15) is a layer of insulating material (e.g., SiO₂, SiN₄, or apolymer) which is deposited or induced to form on the surface of thesubstrate on which the conductive layer will be deposited andnanoelements will be assembled. The thickness of the first insulatinglayer is, for example, in the range from about 10 nm to about 10 μm, orabout 20 nm to about 1 μm, or about 30 nm to about 500 nm, or about 5 nmto about 500 nm, or about 40 nm to about 250 nm, or about 50 nm to about100 nm. The first insulating layer is generally planar in structure andextends over the entire substrate layer, or a portion of the substratelayer. The first adhesion layer prevents current leakage from theconductive layer into the substrate during electrophoretic assembly.Adhesion layer (20) is an optional layer deposited onto the firstinsulating layer. The adhesion layer provides improved adhesion of theconductive layer to the first insulating layer, so that the conductivelayer remains in place when voltage is applied to the conductive layerduring electrophoretic assembly. Suitable materials for the adhesionlayer include chromium, titanium, titanium dioxide, titanium nitride,tantalum, tantalum nitride, tungsten, and combinations thereof. Thethickness of the adhesion layer can be, for example, from about 3 nm toabout 50 nm. Conductive layer (30) is a layer of conductive metaldeposited on the adhesion layer (if present) or the first insulatinglayer (in embodiments with no adhesion layer). Suitable materials forthe conductive layer include metals such as gold, silver, tungsten,aluminum, titanium ruthenium, copper, and combinations or alloysthereof. The conductive layer has two portions: (i) a planar base layer(thickness from about 50 nm to about 100 μm), and (ii) a plurality ofraised features (40) which extend above the plane of the base layer (forexample, from about 10 nm to about 10 μm in height) and which haveelectrical continuity with one another through the base layer of theconductive layer. Second insulating layer (50) is initially depositedover the entire conductive layer, including the raised features, andthen planarized by chemical mechanical polishing so as to rendercoplanar the upper exposed surfaces of the second insulating layer andthe raised features. The thickness of the second insulating layer canbe, for example, from about 10 nm to about 10 μm and is generally aboutthe same as the height of the raised metal features. In someembodiments, the thickness of the second insulating layer and the raisedfeatures is the same to within +/−1 μm, 100 nm, 10 nm or even 5 nm or 2nm. The second insulating layer fills the spaces between the raisedfeatures and provides electrical insulation in those regions whichinhibits the assembly of nanoelements during electrophoretic assembly.Suitable materials for the second insulating layer include SiO₂, SiN₄,Al₂O₃, organic polymers, and combinations thereof. In order to furtherinhibit nanoelement assembly in the insulated regions, those regions arepreferably coated with a hydrophobic coating (60). The hydrophobiccoating is preferably a self-assembled monolayer (SAM) of an alkylsilane (which covalently bonds to SiO₂ if that material is used in thesecond insulating layer). The silane can be, for example,octadecyltrichlorosilane, or a similar silane having an alkyl chain ofabout 8-24 carbons in length. The preferred thickness of the hydrophobiccoating is one molecule, though it can also be more than one molecule.The purpose of the hydrophobic coating is to prevent the assembly ofnanoelements on the exposed surface of the second insulating layer; assuch, it only needs to render the exposed surface of the secondinsulating layer hydrophobic, and to be selectively bound to the secondinsulating layer and preferably not bound to the exposed surface of theconducting layer, where nanoelements are to be assembled. Thehydrophobic coating has a contact angle of from 90° to 110°, preferablyabout 100°. In contrast, the exposed metal conductive layer surface hasa contact angle of from 15° to 21°, preferably about 18°.

Fabrication techniques for making a damascene template of the inventionare known to the skilled person. Such techniques as micro- andnanopatterning can be carried out by e-beam lithography,photolithography, and nano-imprint lithography. Deposition of metals canbe performed by sputtering, chemical vapor deposition, or physical vapordeposition. Deposition of polymers and resists can be performed by spincoating. SiO₂ as the second insulating layer can be deposited by plasmaenhanced chemical vapor deposition (PECVD). Etching of the secondinsulating layer and metal conductive layer can be by ion milling,ion-coupled plasma (ICP) and reactive ion etching (RIE). Thetwo-dimensional pattern of the raised features of the metal conductivelayer, and correspondingly the pattern of assembled nanoelements, can beany pattern that can be established using lithographic techniques,including linear features that are straight, curved, or intersecting aswell as geometric shapes such as circles, triangles, rectangles, ordots. The raised features can have a width in the range from about 10 nmto about 100 μm, and a length from about 10 nm to several cm (e.g., thefull diameter of a wafer).

The damascene template topography has significant impact on theefficiency and yield of the assembly and transfer processes. Ideally aflat topography is used, which provides a uniform electric field fromedge to the center of the electrodes, with minimal variation andfacilitating uniform assembly (see FIGS. 6A.1-A.3 and 6B.1-B.3). FIG. 1Cshows a plot of the simulated electric field strengths for various leveldifferences between the metal and the insulator (dishing amount). It isevident that as the dishing amount increases, the non-uniformity in theelectric field from the edge to the center of the electrodes alsoincreases. In addition, a non-flat topography can result in uneventransfer, creating indentations on the transfer substrate surface (seeFIGS. 7A.1-A.4 and 7B.1-B.3). To achieve a flat topography, the endpoint detection in the CMP process needs to be precise [20]. Forexample, sufficiently precise control can be achieved by determining thetime period required for CMP based on the associated material removalrate. A top view and cross sectional view of a damascene template afterCMP resulting in a flat topography are shown in FIG. 1A. An opticalimage of a 3-inch damascene template is shown in FIG. 1B, with highresolution SEM images shown as insets.

It is apparent from FIG. 1C that the electric field strengths close tothe electrode and the insulator are of the same order of magnitude. Inaddition any organic contamination removal process, such as cleaningwith piranha solution (a solution containing a mixture of H₂SO₄ andH₂O₂), can increase the surface energy of the metal and that of theinsulator. During an electrophoretic assembly process, a substantialelectric field near the SiO₂ surface in conjugation with SiO₂ having ahigh surface energy can result in nanoelement assembly even onto theSiO₂ surface that is undesired (see FIGS. 8A-8B).

To assemble the nanoelements specifically on the gold electrode surface,the electric field strength near the SiO₂ surface and its surface energyshould be decreased. Reducing the electric field strength throughapplication of lower voltage would also reduce the electric strengthnear the gold surface drastically affecting assembly of nanoelements onthe gold electrode. Alternatively, if the surface energy of SiO₂ isreduced without affecting that of the electrode, assembly can beachieved specifically on the gold electrodes. Self-assembled monolayers(SAMs) can be employed to reduce the surface energy of the SiO₂ surfacesignificantly. A preferred material for preparing a SAM for coating theexposed surfaces of the SiO₂ second insulating layer isoctadecyltrichlorosilane (OTS); OTS can be used to modify the surfaceenergy of the SiO₂ layer without affecting the surface energy of theraised gold features. Application of a SAM consisting essentially of OTSincreased the contact angle of SiO₂ to 100° from an initial value ofless than 10°. A post treatment process was developed to selectivelyremove the physically attached OTS SAM layer from the gold withoutdisturbing the OTS SAM layer on the SiO₂ surface (see FIGS. 9A-9D).

FIG. 2 shows an illustration of an assembly and transfer process using adamascene template of the invention. Electrophoresis is employed toachieve directed assembly of nanoelements, while a transfer printingmethod is employed to transfer the assembled nanoelements onto thesurface of a flexible substrate. The surface-modified template isimmersed into a suspension containing uniformly dispersed nanoelements.The properties of the solution (e.g., pH of an aqueous suspension) areadjusted such that the nanoelements have a charge (negative orpositive). DC voltage is applied between the damascene template (havinga polarity opposite to the charge on the nanoelements) and a bare goldtemplate (having a polarity opposite to that of the damascene template),which acts as counter electrode. For example, alkaline pH can render thenanoelements negatively charged, the damascene template can bepositively charged, and the counter electrode negatively charged.Voltage is applied for a brief period, typically less than one minute(e.g., for a 20 sec. time period). The charged nanoelements areselectively assembled on the electrode surface and not on the insulator.With the potential still being applied, after assembly, the template andthe counter electrode are withdrawn from the suspension with a constantspeed. It is critical to have the potential applied during withdrawal,since the hydrodynamic drag on the assembled nanoparticles is strongenough to remove them if the potential is not applied [21]. Typicalassembly results for nanoparticle assembly are shown in FIGS. 3B-3D.

For a given charge on the nanoelements, the applied voltage between thetemplate and the counter electrode considerably dominates the assemblyefficiency of nanoelements (see FIGS. 10A-10D). For low voltages, theelectric field strength at the electrode edges is strong enough toattract and assemble the nanoelements, while at the center it is not andhence no assembly occurs. The withdrawal speed also has an impact on theassembly efficiency (see FIGS. 11A-11D). For an applied potential of 2V,100 nm silica nanoparticles (suspended in deionized water with pH 10.8,adjusted by addition of NH₄OH) assembled only on the edges of goldwires, as shown in FIG. 3A. An extremely low withdrawal speed (1 mm/min)can be used under these conditions, so that the dynamic drag force onthe particles is insignificant. This is confirmed by the electric fieldcontours simulated by a 3D finite volume modeling software (Flow 3D),shown in FIG. 1C.

When the applied potential was increased to 2.5 V, 100 nm silicaparticles assembled at all regions across the electrodes in thedamascene template, even at a 5 mm/min withdrawal speed, as shown inFIG. 3B. The efficacy and material compatibility of the assembly processwas demonstrated by assembling (i) silica nanoparticles onto complextwo-dimensional patterns (FIG. 3D), (ii) 50 nm polystyrene-latex (PSL)particles (FIG. 3C), and (iii) highly organized dense assemblies of SWNT(FIGS. 3E, 3F). For many applications highly aligned SWNTs are desiredinstead of random networks, because aligned SWNTs avoid percolationtransport pathways and result in minimal junction resistance betweentubes [22-24] due to more surface area overlap. The alignment ofassembled SWNTs depends on the direction and speed of templatewithdrawal. Lower withdrawal speed leads to better alignment, with atrade-off of increased assembly process time.

Damascene templates for assembling nanoelements may involve bothnanoscale and micron scale geometries and employ electrophoresis todrive directed assembly. That is, nanoscale and micron scale electrodescan be patterned on an insulator such that the nanoscale metalelectrodes are connected to micron scale counterparts which are thenconnected to a large metal pad (as shown in FIG. 5A). During assemblyusing previous template designs, when a potential is applied to thelarge pad, there is a large potential drop across the length of thenanowires due to the increased resistivity of the nanoscale features.This potential drop has a significant impact on the assembly results,and can yield a non-uniform assembly on various portions of thetemplate. A typical result is shown in FIG. 5C, in which nanoparticleassembly occurs only on the micron scale electrodes and not on thenanoscale electrodes that are connected to them. With the damascenetemplates of the present invention, however, since all the nanoscale andmicron scale electrodes are connected to the metal sheet underneath theinsulator (FIG. 5B), when a potential is applied to the metal sheetduring nanoelement assembly there is negligible variation in theelectric potential between the micron scale and nanoscale electrodes.Equivalent resistor circuits are shown for both the conventionaltemplate as well as the damascene template.

Flow 3D software (v.10) from Flow Science, Inc. was used to simulate theelectric field contours for various template dimensions. The inputparameters were: (i) applied voltage 2.5 V, (ii) conductivity, (iii) pH10.8, (iv) insulator thickness 150 nm, (v) dielectric constants for theinsulator and the solution (4 and 80, respectively), and (iv) Mesh size5 nm and 100 nm for distance less than 1 micron and greater than 1micron respectively. The effective electric field contours weregenerated at a distance of 25 nm from the surface. Shown in FIGS.6A.1-6A.3 are the electric filed simulation results for various non-flattopographies of the damascene templates. As the topography approachesbeing flat, the non-uniformity in the electric field strength across themetal electrode decreases. Shown in FIGS. 6B.1-6B.3 is the simulatedresult for a conventional template in which the nanoscale electrode isnot connected to a metal electrode underneath. Due the morphology andtopology variation, the non-uniformity of the electric field across theelectrode is highly pronounced, which can lead to assembly only at theedges of the electrode.

The assembled nanoelements were then transferred onto flexible polymersubstrates (e.g., PEN, PC) using a nanoimprint tool. The transferefficiency of the transfer printing process is primarily determined bythe differential adhesion force between subject (nanoelements)/template(ST) and nanoelement/recipient (SR). If the adhesion force betweennanoelement and template, FST, is smaller than the adhesion forcebetween nanoelement and recipient, FSR, the nanoelements will betransferred onto the recipient surface. If the contrary is true, thenanoelements will remain on the template surface after the transferprocess [18]. During transfer, the OTS SAM hydrophobic coating on theSiO₂ layer plays the additional role of being an anti-stiction layerwhen the damascene template is separated from the flexible substrateduring transfer. This transfer process does not significantly affect theOTS layer and hence the surface energy of SiO₂, which enables thedamascene template to be reused for assembly-transfer cycle withoutadditional surface modification for several hundred cycles (see FIG.12). Also, no additional processes such as stripping, patterning, orsacrificial layer removal/deposition are needed.

In general, the contact angle of the polymer films used to coat thesecond insulating layer is ˜70°, which is very close to beinghydrophobic and hence low surface energy. In order to improve theadhesion between assembled nanoelements and the polymer film (FST), thepolymer film was pretreated using oxygen plasma in an inductivelycoupled plasmatherm before the transfer printing process was carriedout. This procedure results in the creation of hydroxide groups on thepolymer surface, thereby increasing the surface energy of the polymerfilm [25] [26]. After surface treatment, the contact angle of thepolymer film was found to be less than 5°. For the transfer printingprocess of the invention, a process temperature of about 160° C. wasmaintained, while a pressure of 170 psi was used. This temperature isslightly higher than the glass transition temperature of the polymerfilm (155° C. for PET and 150° C. for PC) and is required to engulf theassembled nanoelements, such that a complete transfer can be achieved[19] (see FIGS. 13A-13C). To measure the electrical properties of thesetransferred SWNTs metal electrodes were fabricated by standardmicrofabrication processes. FIGS. 4C.1-4C.4 show the I-V measurement ofthe transferred SWNTs (2.4 μm channel width) on a PEN film as functionof channel length. The measured resistance was 3.2 kΩ and 12.2 kΩ forchannel lengths of 2 μm and 17 μm respectively. FIG. 4D exhibits therobustness of the assembled SWNT structure under bending. The resistanceincreases linearly as a function of bending radius with a maximum changeof 13% compared to that of the initial value (see FIG. 14).

During transfer of assembled nanoelements using a template with non-flattopography (i.e., having raised metal features) onto a flexiblesubstrate, the transfer might be expected to be partial rather thancomplete. Another possible result is the creation of imprintedstructures (replica of the template) on the flexible substrate. In manycases this is not a desired result, since subsequent processing (metaldeposition, etching, etc) can yield devices with non-uniformcharacteristics. Such an observed result using a non-flat topographydamascene template is shown in FIGS. 7A.1-7A.4 and 7B.1-7B.3.

When electrophoretic assembly was carried out without an OTS SAM layeron the exposed surfaces of the second insulating layer, the nanoelementsassembled everywhere including the insulator region as well as theconductor region (electrode). Without the OTS SAM layer, the surfaceenergies of the insulator and the electrode are approximately the sameas shown in FIG. 8A. When the potential is applied to the metal sheetunderneath the insulator, the potential drop across the insulator isinsufficient to prevent nanoelement assembly, and hence the nanoelementsassembled on the insulator, decreasing the selectivity of the assemblyresult. A typical result for nanoparticle and SWNT assembly without anOTS SAM layer is shown in FIG. 8B.

The OTS SAM layer is applied to the damascene template using a wetchemical method. During this process an OTS SAM layer also can form onthe metal electrodes and it can inhibit nanoelements from beingassembled on them. In order to remove the OTS SAM layer selectively fromthe metal electrode, a chemical treatment with “piranha solution” wasperformed on the damascene template. The piranha treatment removed onlythe OTS SAM that was present on the metal electrodes and left themonolayer on the insulator unaffected. This was verified through contactangle measurements as shown in FIGS. 9A-9D before and after piranhatreatment following OTS SAM layer deposition on the damascene template.

SWNTs used for assembly have terminal carboxylic acid groups due totheir purification process. When suspended in deionized water, thesecarboxylic acid groups impart a negative charge to the SWNTs atsufficiently high pH. The electrophoretic force on the nanoelements dueto an applied potential is directly proportional to the charge on thenanoelements and the electric field strength. When the applied voltageis increased, the electrophoretic force increases proportionally,resulting in increased amount of nanoelements assembled on the metalelectrodes. FIGS. 9A-9D clearly show the significant effect of voltageon SWNT assembly. It can be seen from these results that assembly ofSWNTs on the electrodes started between 1.5 V and 2 V. Beyond a criticalvalue of the applied potential, the barrier introduced by SiO₂ fails,and nanoelements can assemble on the insulator surfaces as shown inFIGS. 10A-10D.

The capillary force acting on assembled nanoelements during theirwithdrawal from the suspension after assembly plays a crucial role onthe adhesion of the nanoelements to the metal electrodes on thedamascene template. For higher withdrawal speeds the removal momentacting on the nanoelements due to the capillary force would be larger,resulting in removal of the nanoelements. For a given type ofnanoelement and applied potential the withdrawal speed needs to beadjusted, and can be characterized as illustrated in FIGS. 11A-11D. Theadhesion force can be further improved by keeping the applied potentialon. In all of these experimental results shown herein, the potential waskept on during the template withdrawal process.

Failure of the assembly and transfer process using damascene templatesafter several assembly and transfer cycles would be expected if the OTSSAM layer deteriorates on the insulator surfaces. If the OTS SAM on theinsulator deteriorates then nanoelements can assemble on the insulator,resulting in low yield. To test the versatility and robustness of thedamascene templates, the contact angle of the insulator surface wasmeasured after each assembly and transfer cycle, and it is plotted inFIG. 12. Extrapolation of these results based on the assumption that theOTS SAM layer deteriorates at the same rate in subsequent assembly andtransfer cycles leads to the estimate that the contact angle would reacha value of 70° after 140 cycles while it would reach a value of 50° atabout 250 cycles. The contact angle of the metal electrode would alsoincrease as a function of the number of cycles, and will eventuallysaturate. If one assumes the saturated contact angle value of 50° thenthe life cycle for a single coat of OTS SAM layer would be about 250cycles. When the OTS SAM layer has deteriorated, another layer of OTSSAM layer can be added to the template, and it can be reused again forassembly and transfer.

The temperature applied to the substrates during the transfer processhas an important effect on the transfer efficiency. The transfer processtemperature preferably is close to that of the glass transitiontemperature of the polymer that makes up the receiving substrate. Thisis demonstrated in FIGS. 13A-13C. From the figure it is clear that whenthe process temperature is raised beyond the Tg for PEN (155° C.) theassembled nanoelements are transferred completely, achieving essentially100% transfer yield.

The flexible recipient substrate with transferred nanoelements wassubjected to a bending test. Cylindrical objects such as those shown inthe inset of FIG. 14 were used for the bending test. The PEN film withtransferred SWNTs and deposited electrodes was taped to thecircumference of the cylindrical object and resistance measurements weretaken in the bent state. The results are shown in FIG. 14.

As used herein, “consisting essentially of” does not exclude materialsor steps that do not materially affect the basic and novelcharacteristics of the claim. Any recitation herein of the term“comprising”, particularly in a description of components of acomposition or in a description of elements of a device, can beexchanged with “consisting essentially of” or “consisting of”.

While the present invention has been described in conjunction withcertain preferred embodiments, one of ordinary skill, after reading theforegoing specification, will be able to effect various changes,substitutions of equivalents, and other alterations to the compositionsand methods set forth herein.

REFERENCES

-   [1] T. Kraus, L. Malaquin, H. Schmid, W. Riess, N. D. Spencer, H.    Wolf, Nature nanotechnology 2007, 2, 570.-   [2] C. Yilmaz, T. H. Kim, S. Somu, A. A. Busnaina, Nanotechnology,    IEEE Transactions on 2010, 9, 653.-   [3] R. Krupke, F. Hennrich, H. Weber, M. Kappes, H. Löhneysen, Nano    letters 2003, 3, 1019.-   [4] P. Maury, M. Escalante, D. N. Reinhoudt, J. Huskens, Advanced    Materials 2005, 17, 2718.-   [5] Y. Xia, Y. Yin, Y. Lu, J. McLellan, Advanced Functional    Materials 2003, 13, 907.-   [6] L. Jaber-Ansari, M. G. Hahm, S. Somu, Y. E. Sanz, A.    Busnaina, Y. J. Jung, Journal of the American Chemical Society 2008,    131, 804.-   [7] X. Xiong, P. Makaram, A. Busnaina, K. Bakhtari, S. Somu, N.    McGruer, J. Park, Applied physics letters 2006, 89, 193108.-   [8] R. C. Bailey, K. J. Stevenson, J. T. Hupp, Advanced Materials    2000, 12, 1930.-   [9] Q. Zhang, T. Xu, D. Butterfield, M. J. Misner, Du Yoel Ryu, T.    Emrick, T. P. Russell, Nano letters 2005, 5, 357.-   [10] E. Kumacheva, R. K. Golding, M. Allard, E. H. Sargent, Advanced    Materials 2002, 14, 221.-   [11] B. Li, H. Y. Jung, H. Wang, Y. L. Kim, T. Kim, M. G. Hahm, A.    Busnaina, M. Upmanyu, Y. J. Jung, Advanced Functional Materials    2011, 21, 1810.-   [12] J. H. Ahn, H. S. Kim, K. J. Lee, S. Jeon, S. J. Kang, Y.    Sun, R. G. Nuzzo, J. A. Rogers, science 2006, 314, 1754.-   [13] Y. Sun, H. H. Wang, Advanced Materials 2007, 19, 2818.-   [14] D. Lee, T. Cui, Biosensors and Bioelectronics 2010, 25, 2259.-   [15] B. Li, M. G. Hahm, Y. L. Kim, H. Y. Jung, S. Kar, Y. J. Jung,    ACS nano 2011, 5, 4826.-   [16] M. A. Meitl, Z. T. Zhu, V. Kumar, K. J. Lee, X. Feng, Y. Y.    Huang, I. Adesida, R. G. Nuzzo, J. A. Rogers, Nature Materials 2005,    5, 33.-   [17] F. N. Ishikawa, H. Chang, K. Ryu, P. Chen, A. Badmaev, L. Gomez    De Arco, G. Shen, C. Zhou, ACS nano 2008, 3, 73.-   [18] D. Hines, V. Ballarotto, E. Williams, Y. Shao, S. Solin,    Journal of applied physics 2007, 101, 024503.-   [19] T. Tsai, C. Lee, N. Tai, W. Tuan, Applied physics letters 2009,    95, 013107.-   [20] T. Bibby, K. Holland, Journal of electronic materials 1998, 27,    1073.-   [21] S. Siavoshi, C. Yilmaz, S. Somu, T. Musacchio, J. R.    Upponi, V. P. Torchilin, A. Busnaina, Langmuir 2011, 27, 7301.-   [22] E. Artukovic, M. Kaempgen, D. Hecht, S. Roth, G. Grüner, Nano    letters 2005, 5, 757.-   [23] L. Hu, D. Hecht, G. Grüner, Nano letters 2004, 4, 2513.-   [24] M. Fuhrer, J. Nygård, L. Shih, M. Forero, Y. G. Yoon, H. J.    Choi, J. Ihm, S. G. Louie, A. Zettl, P. L. McEuen, science 2000,    288, 494.-   [25] N. Inagaki, Plasma surface modification and plasma    polymerization, CRC, 1996.-   [26] E. Liston, L. Martinu, M. Wertheimer, Journal of adhesion    science and technology 1993, 7, 1091.

The invention claimed is:
 1. A method of assembling and transferring atwo-dimensional patterned assembly of nanoelements onto a flexiblepolymer substrate, the method comprising the steps of: (a) providing ananoelement transfer system and a liquid suspension of nanoelements, thenanoelement transfer system comprising (i) a nanoelement assembly devicefor creating a patterned assembly of nanoelements; (ii) a thermallyregulated imprint device for applying pressure between the damascenetemplate and a flexible polymer substrate at a selected temperatureabove ambient temperature for transfer of said patterned assembly ofnanoelements onto said flexible polymer substrate; and (iii) a damascenetemplate comprising a substantially planar substrate; a first insulatinglayer disposed on a surface of the substrate; an optional adhesion layerdisposed on a surface of the first insulating layer opposite thesubstrate; a conductive metal layer disposed on a surface of theadhesion layer opposite the first insulating layer, or disposed on asurface of the first insulating layer opposite the substrate if theadhesion layer is absent; a second insulating layer disposed on asurface of the conductive metal layer opposite the adhesion layer, oropposite the first insulating layer if the adhesion layer is absent; anda hydrophobic coating selectively disposed on exposed surfaces of thesecond insulating layer opposite the conductive metal layer; wherein theconductive metal layer is continuous across at least one region of thesubstrate, and within said region the conductive metal layer has atwo-dimensional microscale or nanoscale pattern of raised features thatinterrupt the second insulating layer; wherein the second insulatinglayer substantially fills the spaces between said raised features; andwherein exposed surfaces of the raised features and the exposed surfacesof the second insulating layer are essentially coplanar; (b) submergingthe damascene template in the liquid suspension of nanoelements; (c)applying a voltage between the conductive metal layer of the submergeddamascene template and a counter electrode in the liquid suspension,whereby nanoelements from the suspension are assembled onto the exposedsurfaces of the raised features of the conductive metal layer of thedamascene template and not onto the exposed surfaces of the secondinsulating layer of the damascene template, thereby forming a patternedassembly of nanoelements on a surface of the damascene template; (d)withdrawing the damascene template and attached patterned assembly ofnanoelements from the liquid suspension while continuing to applyvoltage between the conductive metal layer of a submerged portion of thedamascene template and the counter electrode; (e) drying the withdrawndamascene template; and (f) contacting the patterned assembly ofnanoelements attached to the damascene template with the flexiblepolymer substrate and applying pressure and heat using the thermallyregulated imprint device, whereby the patterned assembly of nanoelementsis transferred onto the flexible polymer substrate; wherein thecontacting is performed at a temperature above the glass transitiontemperature of the flexible polymer substrate.
 2. The method of claim 1,wherein during steps (c) and (d) the conductive metal layer of thedamascene template is positive, the counter electrode is negative, andthe pH of the liquid suspension is adjusted such that the nanoelementshave a negative charge.
 3. The method of claim 1, wherein during steps(c) and (d) the conductive metal layer of the damascene template isnegative, the counter electrode is positive, and the pH of the liquidsuspension is adjusted such that the nanoelements have a positivecharge.
 4. The method of claim 1, wherein the voltage applied in steps(c) and (d) is sufficiently high to assemble the nanoelements from thesuspension across essentially the entire exposed surface of the raisedfeatures of the conductive metal layer of the submerged damascenetemplate.
 5. The method of claim 1, wherein a speed of withdrawing thedamascene template in step (d) is sufficiently slow to retain theattached patterned assembly of nanoelements on the surface of the raisedfeatures of the conductive metal layer of the damascene template throughthe withdrawal process.
 6. The method of claim 1, wherein the voltage insteps (c) and (d) is in the range from 1.5 to 7 V and a speed ofwithdrawing in step (d) is in the range from 1 to 15 mm/min.
 7. Themethod of claim 1, wherein steps (b) through (f) are repeated one ormore times using the same damascene template and one or more additionalflexible polymer substrates, whereby a plurality of patterned assembliesof nanoelements are transferred onto the additional flexible polymersubstrates.
 8. The method of claim 7, wherein steps (b) through (f) arerepeated for a total of about 250 cycles and about 250 flexible polymersubstrates are produced, each comprising a substantially identicalpatterned assembly of nanoelements.
 9. The method of claim 8, whereinthe hydrophobic coating of the damascene template is refreshed, followedby performing up to an additional 250 cycles of steps (b) through (f).